Thaw cycle in condensing style gas furnaces

ABSTRACT

A condensing gas furnace having an inducer for drawing exhaust produced by a burner through an exhaust path and venting the exhaust from the furnace. A pressure sensor positioned within the exhaust path to determine if the inducer is creating a negative pressure in the exhaust path preventing accumulation of exhaust within the exhaust path. The inducer includes a supplemental sensor for monitoring rotation of a fan of the inducer. The burner is not ignited when both sensors fail to indicate proper venting of the exhaust path. The burner is operated for a predetermined period of time when only the supplemental sensor signals rotation of the fan to thaw any condensate on the supplemental sensor.

BACKGROUND

Heating, ventilating, and air conditioning (HVAC) equipment is used to heat, cool, and ventilate buildings and other enclosed spaces where people live and work. Air conditioning units are commonly used to provide cooling in the summer months. In addition, furnaces can be packaged separately or with air conditioning units and the furnaces are commonly operated in the winter months to provide heating. Furthermore, condensing furnaces have been used to reduce consumption of fossil fuels (e.g., natural gas or propane) burned in furnaces to provide heating. Condensing gas furnaces are often located indoors. Some buildings, however, are configured to have an HVAC unit installed on the roof of the building or on the ground outside the building. In such situations, condensate that accumulates in a condensing gas furnace can freeze, which, in turn, can present a number of challenges to proper operation of the furnace.

Certain condensing style gas furnaces are employed with the primary heat exchanger elements located outside of an enclosed space, such as on the roof of a building. Many gas furnaces include a pressure switch as a safety sensor for detecting whether products of combustion generated by the burners within the furnace are properly exhausted. Specifically, the pressure switch is typically operably connected to an inducer fan operable to vent exhaust from the burners when the pressure exceeds a safe threshold. In colder climates, the passage that communicates inducer airflow to the pressure switch can freeze-over and thereby restricting or blocking the airflow to the pressure switch. When the pressure switch conduit is blocked, the controller for the furnace does not receive a signal from the pressure switch indicating the exhaust is being properly vented and the pressure within the HVAC system is within safe tolerances. Accordingly, controllers of such furnaces may simply extinguish the burner and shut down the furnace when the pressure sensor signal is not received.

OVERVIEW

In an example, the condensing gas furnace can include a burner, an igniter, a gas valve, a primary and secondary heat exchanger, a pressure sensor, a supplemental sensor, and a controller for receiving and monitoring signals from the various sensors, inducer, blower, and a thermostat. The burner can produce the products of combustion, such as by igniting fuel.

In an example, the controller can be started upon receiving a signal from a thermostat that indicates that the temperature within an enclosed space heated by the furnace is below a minimum threshold temperature. Upon receiving the signal, the controller can start an inducer motor to rotate a fan to draw un-burned products of combustion from the furnace. In normal operation, the controller can receive at least two sensor signals before igniting the burner: (1) an indication of a negative pressure along the exhaust path, and (2) an indication from a supplemental sensor that the inducer fan is rotating. The pressure signal verifies the inducer fan is rotating at a rotational speed sufficient to produce a pressure differential that can exhaust the products of combustion from the furnace. Occasionally, in cold weather climates, the pressure sensor port can freeze over, which can provide a false indication that the inducer is not operating properly and falsely signals the burner to remain off.

In an example, the thaw cycle can restore operation of a lost pressure sensor signal by melting the frozen condensate blocking the pressure sensing port. The furnace may be run for short periods of time without causing a safety concern as long as the inducer fan is operating properly. If the controller receives an indication from the supplemental sensor that the inducer is running, but the pressure signal is not received, the thaw cycle can be initiated in an attempt to restore operation. During the thaw cycle, the inducer is started during the thaw cycle and after a period of time an ignition cycle is commenced. The gas valve can be opened and the burner is ignited to generate exhaust in the exhaust path. If a flame sensor detects ignition, the thaw cycle will run for one-minute providing heat to the heat exchangers, inducer, and outlet. If the controller still fails to receive a signal from the pressure sensor, the thaw cycle can repeat up to five-times. A one hour soft-lock period will begin if the controller does not receive a pressure signal after the fifth thaw cycle. If the controller receives both the Hall sensor and the pressure sensor signals, the furnace can remain in operation, such as until the call for heat is no longer received from the thermostat.

Despite the various efficiency and environmental benefits of condensing gas furnaces, freezing condensate can prevent or inhibit the installation of such devices in outdoor applications. Without confirmation that the inlet or outlet of the inducer is clear of any blockage, such furnaces would typically need to be shut down for safety reasons. Where the pressure sensor is merely frozen, such a shutdown may be unnecessary. Examples according to this disclosure address the issue of unnecessary shutdowns by implementing thaw cycles to resolve safety sensor/switch signal loss, while maintaining safe operation of the furnace.

This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the present subject matter. The detailed description is included to provide further information about the present patent application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.

FIG. 1 is a schematic of a gas furnace, according to an example, according an example of the present subject matter.

FIG. 2 is a perspective view of a heat exchange and inducer exhaust assembly, according an example of the present subject matter.

FIG. 3 is a perspective view of a furnace inducer including a pressure sensor port and supplemental sensor, according to an example of the present subject matter.

FIG. 4 is a flow chart illustrating a method for operating a gas furnace, according to an example of the present subject matter.

FIG. 5 is a flow chart illustrating a process for starting a gas furnace in cold weather, according to an example of the present subject matter.

DETAILED DESCRIPTION

The present disclosure is related to methods and apparatuses for operating HVAC systems in cold weather conditions, where frozen condensate can prevent safety sensors from functioning properly. Specifically, the present disclosure relates to methods and apparatuses for performing a “thaw cycle” to remove frozen condensate from the pressure sensor passage and prevent interference with the operation of the pressure sensor without unnecessary shut down of the furnace in cold weather applications. This technique is sometimes referred to collectively as a “thaw cycle,” which can refer to all of the necessary and optional functional steps and associated structure and components for carrying out such steps in accordance with examples of this disclosure.

As depicted in FIG. 1, a gas furnace 100, according to an example, includes a burner 102, an inducer 104, a primary heat exchanger 110, a secondary or “condensing” heat exchanger 124, a blower 126, and a controller 116. The furnace 100 can comprise a high-efficiency condensing furnace configured to transfer heat from the burner 102 and heat exchangers 110 and 124 to a working fluid 122, such as air. The blower 126 is configured to draw the working fluid 122 across the primary and secondary heat exchangers 110, 124 to heat the working fluid 122. The heated working fluid 122 can then be circulated through a plurality of ducts to within an enclosed space 120, such as an interior portion of a building or other enclosed structure, by the blower 126. The blower 126 can include a fan, a motor, and a housing, such as to draw air across the primary heat exchanger 110 or the secondary heat exchanger 124. Heat from the primary heat exchanger 110 and secondary heat exchanger 124 can be transferred to heat the air that is circulated into the space 120.

The burner 102 can be configured to ignite fuel to produce an exhaust containing products of combustion. The burner 102 can include a hot surface igniter or an igniter configured to produce a spark or series of sparks to ignite a stream of vaporized fuel gas from a gas valve. In certain examples, the igniter is configured to continue glowing or producing sparks until the burner 102 is ignited. In this configuration, the igniter can be timed to cease glowing or sparking after a predetermined time. Alternatively, the igniter can be configured to cease sparking upon receiving a signal from a flame sensor indicating that the burner 102 has been ignited.

The inducer 104 includes a fan rotatable to draw the products of combustion along an exhaust path 106 to an outlet 108. A motor can be coupled to the fan of the inducer 104. The motor can be configured to provide torque to the fan, such as to rotate the fan in order to exhaust the products of combustion. The motor can rotate the fan at a sufficient speed to achieve a desired flow rate of the products of combustion through the exhaust path 106. In an example, a flow rate that can exhaust the products of combustion at a rate equal to or greater than the rate they are created by the burner 102.

The primary heat exchanger 110 is positioned along the exhaust path 106 and includes an inlet for receiving the exhaust from the burner 102. The exhaust is circulated within the primary heat exchanger 110 to heat the working fluid 122. The working fluid 122 stream is separated from the exhaust by a heat conductive wall such as a pipe to allow transfer of heat between the working fluid 122 and the exhaust without intermixing the streams. The temperature of the working fluid 122 can be increased by the heat exchanger 110 such as by passing the working fluid 122 over the surface of the heat exchanger 100. The exhaust exits the primary heat exchanger 110 and enters an inlet of the secondary heat exchanger 124. The furnace 100 can gain further efficiency by transferring additional heat remaining in the products of combustion to the working fluid 122 by virtue of the secondary heat exchanger 124. The secondary heat exchanger 124 can be configured so that a portion of the products of combustion condense while passing through the secondary heat exchanger 124. Through the phase change from a gas to a liquid, further heat transfer can occur.

In an example, the primary heat exchanger 110 and the secondary heat exchanger 124 can be configured to maximize the surface area in contact with the working fluid 122, such as to maximize the heat transfer from the products of combustion to the working fluid 122. In an example, the primary heat exchanger 110 and the secondary heat exchanger 124 can comprise an S-shape, U-shape, or finned geometry. The outlet 108 can be a conduit for exhausting the products of combustion from the furnace 100. In an example, the outlet 108 can be a pipe sized and shaped to facilitate the desired flow rate of the exhaust though the exhaust path 106. The outlet 108 can include a drain, such as for removing liquid condensate produced by the cooling of the products of combustion from the furnace 100.

In an example, the gas furnace 100 can also include a controller 116 can include at least one processor and a non-transitory computer readable medium with instructions configured thereon for controlling various functions of the furnace 100. The functions that can be regulated by the controller 116 can include the operation of the inducer 104, a gas valve, one or more timers, an igniter, the blower 126, a counter, and other functions. The controller 116 can be configured on a single printed circuit board.

In an example, the gas furnace 100 can also include a pressure sensor 112 positioned along an exhaust path 106. In certain examples, the pressure sensor 112 is positioned on the inducer 104. The pressure sensor 112 is configured to detect a pressure differential from in the exhaust path 106 and transmit a signal to the controller 116 indicating whether the fan of the inducer 104 is operating at speed sufficient to safely run the furnace 100. The pressure sensor 112 can be configured to detect a positive or negative pressure depending upon the location of the pressure switch along the exhaust path 106. The pressure sensor 112 can include a diaphragm switch, a differential switch, a sail switch, or other. The pressure sensor 112 can measure the relative pressure between the atmospheric pressure and the measured pressure in the exhaust path 106. In this configuration, the pressure sensor 112 can be configured to detect operation of the inducer 104 to exhaust the products of combustion produced by burner 102. Insufficient pressure in the exhaust path 106 can indicate the inducer 104 is not operating or not drawing the products of combustion through the exhaust path 106 at a rate greater than the rate at which the products of combustion are produced by the burner 102). A blocked outlet 108 can reduce the rate in which the products of combustion are exhausted from the furnace 100. If the products of combustion are not exhausted at a sufficient rate, the furnace 100 may exceed its temperature operating limits.

In an example, the pressure sensor 112 can include a relay such that the pressure sensor 112 functions as a pressure switch. The relay can include a switch that can open or close in response to the pressure sensor 112 detecting a threshold pressure value. The threshold pressure value corresponds to a pressure value that indicates sufficient exhaust rate of the products of combustion).

In an example, the furnace 100 can include a supplemental sensor 114, such as a Hall Effect sensor. The supplemental sensor 114 is configured to detect the rotational speed of the inducer fan. The rotational speed of the fan can indicate whether the products of combustion are exhausted at a rate sufficient to safely operate the furnace 100. In this configuration, the pressure sensor 112 and supplemental sensor 114 can be communicatively coupled to a controller 116 for controlling and monitoring functions of the furnace 100.

In an example, the supplemental sensor 114 can be configured to detect the rotation of the fan through the housing 302, such as a Hall Effect sensor that can detect the frequency at which a magnetic field (e.g., generated by the rotation of an alternating current (AC) motor) comes within detection range of the supplemental sensor 114.

In an example, the supplemental sensor 114 can include a centrifugal switch mountable on a fan motor, a sail switch mountable along the exhaust path 106, and a current sensing relay configured to activate when a motor current of the inducer exceeds a threshold. The relay can include a switch that can open or close in response to the supplemental sensor 114 detecting a threshold rotational speed of the fan. In an example, the threshold rotational speed can comprise 2,000 rpm. In an example, the relay can have a hysteresis to reduce de-activation of the inducer motor 304 when the rotational speed of the fan is fluctuating near the first threshold value. Once the relay has closed the switch, the relay will not re-open the switch until the fan speed drops below a second threshold. In an example, the second threshold rotational speed can comprise 1,800 rpm. In this configuration, the switch is normally in an open configuration such that the controller 116 can only position the gas valve in the fully open position and ignite the burner 102 when the switch is switched to a closed configuration. In an example, the supplemental sensor 114 can be modified from the open configuration to the closed configuration when the rotation of the fan of the inducer 104 exceeds a first threshold rotational speed. The first threshold rotational speed corresponds to a sufficient speed to exhaust the products of combustion. If the inducer 104 is not exceeding the first threshold rotational speed, the switch is maintained in the closed configuration preventing opening of the gas valve and ignition of the burner 102.

In an example, the inducer 104 is communicatively coupled to the controller 116, the controller 116 being configured to initiate or cease rotation of the fan of the controller 116. The controller 116 can also be configured to start or stop rotation of the fan in response to signals transmitted from the pressure sensor 112 or the supplemental sensor 114.

In an example, a thermostat 118 can be positioned within the enclosed space 120 and configured to detect a temperature within the enclosed space 120. The thermostat 118 is configured to compare the measured temperature within the enclosed space 120 with a target temperature. The thermostat 118 can be communicatively coupled to the controller 116. In operation, the thermostat 118 can include a processor and a non-transitory computer readable medium with instructions for signaling the controller 116 that the temperature within the enclosed space 120 is less than a set point temperature. In this configuration, the controller 116 is communicatively coupled to the burner 102 and configured to initiate operation of the burner 102 when the notified by the thermostat 118 that the temperature within the enclosed space 120 is below a minimum threshold temperature or exceeds a maximum threshold temperature. In an example, the controller 116 can start the blower 126 in response to the notification from the thermostat 118 that the temperature is below the minimum threshold temperature.

In an example, the gas valve can be configured to regulate the volume of gas to be consumed in the burner. In an example, the gas valve can include a solenoid that can be actuated to define at least a high flow position, a low flow position, and a closed position. In an example, the valve can be opened to the high flow position for ignition. The burner 100 can be turned off by actuating the solenoid to the closed position to cutoff the gas flow. In an example, the gas valve can be communicatively coupled to the controller 116. In this configuration, the controller 116 is configured to be notified by the thermostat 118 that the temperature within the enclosed space 120 exceeds the maximum threshold temperature. The controller 116 is further configured to actuate the solenoid to set the gas valve to the closed position and turn off the burner 102.

The thermostat 118 can send instructions to the controller 116, such as a call for heat. A call for heat can be generated in response to the temperature in the space 120 being below a target temperature. In an example, the thermostat 118 can be located within the space 120. The call for heat can be interrupted in response to the temperature within the space 120 exceeding the target temperature. The controller 116 can continuously monitor the output of the thermostat 118. Additionally, the thermostat 118 can signal to the controller 116 to open the gas valve to a high or low flow position.

As depicted in FIG. 2, in an example, the furnace 100 can include one or more burners 102 mounted to a support structure, such as a partition 202. The partition 202 can be an interior wall within the furnace 100. The partition 202 can separate the flowing working fluid 122 from the outside air or air used as input for combustion.

In an example, the furnace 100 can include a gas manifold 204 for supplying fuel to the one or more burners 102 as shown. The fuel flow provided to the one or more burners 102 by the gas manifold 204 can be regulated by a gas valve 206. The gas valve 206 can be connected to a gas supply, such as a municipal gas line or gas storage tank. The gas manifold 204 can distribute the gas from the gas supply to the one or more burners 102, such as through a series of equally sized outlets that are coupled to the inlet of each burner 102.

In example, each of the one or more burners 102 are oriented to direct the flame into the inlet of the primary heat exchanger 110, such as the inlet of the heat exchanger 110 can directly receive the exhaust from the burners 102. The inlet of heat exchanger 110 can be supported by the partition 202. In an example, the inlet of the secondary heat exchanger 124 can be coupled to the outlet of the heat exchanger 110, such that the products of combustion can pass from the heat exchanger 110 through the secondary heat exchanger 124. In this configuration, the primary heat exchanger 110 and the secondary heat exchanger 124 can be coupled through a header assembly 208 as depicted in FIG. 2. Condensate from the secondary heat exchanger 124 can drain into the header 208 that couples the primary heat exchanger 110 and the secondary heat exchanger 124 or drain into a collector 210.

In an example, the collector 210 can be coupled to the outlet of heat exchangers 124. The partition 202 can support the outlet of the secondary heat exchanger 124 and the collector 210 to position the outlet of the heat exchanger 124 with the inlet of the collector 210 such that the condensed portion of the products of combustion can accumulate in the collector 210. The collector 210 is operably coupled to a drain line 212 for draining the condensed products of combustion collected from the collector 210 from the furnace 100. Draining the condensate can help to maintain a clear exhaust path 106 by removing liquid that may become frozen to block the exhaust path 106.

In an example, the inducer 104 can be coupled to the outlet of the collector 210. In this configuration, the fan of the inducer 104 can be rotated to draw the products of combustion from the burner 102 through the heat exchanger 110, secondary heat exchanger 124, collector 210, and exhaust the products of combustion through the outlet 108.

As depicted in FIG. 3, an inducer 104, according to an example, can include a housing 302 with an inducer inlet (not shown) and inducer outlet 306, a fan (not shown), a motor 304 for driving the fan, a supplemental sensor 114, and a passage 308 in the housing 302 for mounting a sensor, such as a pressure sensor 112. The motor 304 can be coupled to the fan. The motor can include a shaft. The shaft can support the fan, such as in a position to rotate within the housing 302 without interference. The fan can be keyed to the shaft, such as to rotate the fan at the same rate as the shaft when the motor 304 is running. The motor 304 can rotate the fan in response to receiving a signal form the controller 116 to start the inducer 104. When rotating, the fan can be configured to draw the products of combustion from the heat exchanger 110 or secondary heat exchanger 124 and exhaust the products of combustion from the furnace 100 through the inducer outlet 306 and furnace outlet 108. In an example, the rotating fan can generate a positive pressure at the outlet 306 or a negative pressure at passage 308. The positive pressure can propel the products of combustion from the furnace 100. The pressure sensor 112 can be configured to detect the pressure of the products of combustion along the exhaust path 106, such as at the location of the passage 308. A negative pressure can be produced by the fan at the inlet of the inducer 104, such as to draw the products of combustion into the inducer 104.

In an example, the motor 304 can comprise a two-speed motor configured to exhaust the products of combustion at a low rate or a high right. The high speed can exhaust a greater volume of the products of combustion during a period of time than the low speed. In an example, the low rate can comprise 2,000 rpm and the high rate can comprise 3,400 rpm.

In an example, the housing 302 of the inducer 104 can support the motor 304 and the supplemental sensor 114. The housing 302 can direct the products of combustion from the inducer inlet to the inducer outlet 306 if the fan is rotating. A passage can be included within the housing 302, such as for communicating a pressure along the exhaust path 106 to the pressure sensor 112. The housing 302 can also include a bracket (not shown) for mounting a pressure sensor 112 to the housing 302 such that the pressure sensor 112 can receive a signal through the pressure sensor port 308. The passage 308 can become blocked with frozen condensate in cold conditions. In an example, the supplemental sensor 114 can be configured to detect the rotation of the fan through the housing 302, such as a Hall Effect sensor that can detect the frequency at which a magnetic field generated by the rotation of an AC motor comes within detection range of the supplemental sensor 114.

As depicted in FIGS. 4-5, a method 400 for operating a gas furnace 100, according to an example, through a thaw cycle can include monitoring for frozen condensate. Frozen condensate can prevent sensors within the furnace 100 from operating as designed, such as if the condensate blocks the port 308 to the pressure sensor 112. Condensing gas furnaces can be more efficient than traditional gas furnaces; however, the improved efficiency is derived from extracting more heat from the products of combustion to the point in which the products of combustion condense within the secondary heat exchanger 124. This attribute results in frozen condensate within the furnace, such as when the furnace 100 is located outside of the heated enclosed space 120. Technique 400 can remove the frozen condensate from the one or more sensors within the furnace 100. With the frozen condensate removed, the furnace 100 can operate as intended to provide efficient heating of a space 120.

At 402, the controller 116 can start the inducer 104 in response to receiving a call for heat from the thermostat 118. The inducer 104 can run for a pre-purge time, such as to exhaust any un-combusted gas within the furnace 100. The pressure sensor 112 can detect a pressure differential in the exhaust path 106, such as a negative pressure. The pressure sensor 112 can indicate if the inducer 104 is generating a draft within the exhaust path 106 sufficient to exhaust the products of combustion. If there is frozen condensate within the exhaust path 106, the pressure sensor may not detect the pressure generated by the inducer 104.

At 404, the controller 116 can ignite a burner 102 in response to detecting rotation of the fan of the inducer 104 from the supplemental sensor 114, regardless of whether a signal is transmitted from the pressure sensor 112. A signal from the supplemental sensor 114 can indicate that the inducer is running, such as exhausting the products of combustion. The products of combustion can carry heat along the exhaust path. The frozen condensate can thaw in response to exposure the products of combustion. Operation of the pressure sensor 112 can be restored once the frozen condensate has been removed from the inlet of the pressure sensor 112. If the products of combustion are not exhausted at a sufficient rate, the furnace 100 can overheat and the furnace 100 will shut down.

At 406, the controller 116 can extinguish the burner 102 if the signal from the pressure sensor 112 is not received within a prescribed burn time. Extinguishing the burner 102 can prevent the products of combustion from backing up into the furnace 100 such as if outlet is blocked or the inducer is not exhausting the products of combustion at a rate sufficient to keep pace with the production from the burner 102.

In an example, the controller 116 can initiate the method 400 for starting the furnace 100, such as if the pressure sensor 112 is not transmitting a signal indicating the inducer 104 is generating the desired draft rate such as if the inlet of the pressure sensor 112 is blocked by frozen condensate. The thermostat 118 can transmit a signal calling for heat 502 to the controller 116, such as when the temperature in the space 120 falls below a specified level. Method 400 can be initiated in response to the call for heat 502. The method 400 can include one or more cycles of igniting the burner 102 and running the inducer 104, such as to draft the products of combustion through the furnace 100. The controller 116 can implement one or more sequences of operation. The sequence of operation followed by the controller 116 can be determined by signal inputs from various sources, such as the pressure sensor 112, supplemental sensor 114, a counter, or a timer.

In an example, the controller 116 can include instructions to start the inducer 504 in response to the call for heat 502. The inducer 104 can clear the exhaust path 106 of any un-combusted gas remaining within the furnace 100 prior to ignition, such as to prevent excessive combustion upon ignition. The supplemental sensor 114 can detect fan rotation 506 exceeding the first threshold fan speed. In response to the supplemental sensor 114 detecting the fan rotation 506, the supplemental sensor 114 can close an electrical circuit between the controller 116 and the gas valve 206.

If the supplemental sensor 114 is closed, but no signal is received from the pressure sensor 112 indicating the detection of a threshold exhaust pressure 524. The inducer 104 can include a timer 508 for measuring a purge time 510, wherein the inducer 104 is operated a purge time 510 expires. In an example, the purge time 510 can be about 30 to 60 seconds. In response to the expiration of the purge time 510, the controller 116 can ignite 511 the burner 102. The burner 102 can be operated for a burn time 512. In an example, the burn time 512 can comprise about 30, 60, 120, or 150 seconds. The inducer 104 can continue to operate for a post-purge time 518 to clear the products of combustion from the exhaust path 106. In an example, the post-purge time 518 can comprise about 30, 60, 120, or 150 seconds.

In an example, the controller 116 can include a counter. The number of thaw cycles attempted by the furnace 100 can be tracked by the counter. The counter can be incremented 516 by a single integer following the expiration of the post-purge time 518 marking the completion of a thaw cycle. If the counter has recorded less than a predetermined cycle limit 520 then a subsequent cycle can be initiated. In an example, the predetermined cycle limit 520 can be five. The subsequent cycle can include: detecting fan rotation 506 from the supplemental sensor 114, starting the timer 508, and igniting 511 the burner 102. When the burn-time 512 expires, the burner 102 can be extinguished 514. The inducer 104 can remain in operation until the post-purge time 518 expires. Following the expiration of the post-purge time 518, the counter can be incremented 516. The cycle can repeat until the counter reaches the cycle limit 520 without receiving a signal from the pressure sensor 112, such as a signal detecting a minimum exhaust pressure within the exhaust path 106. The furnace 100 can enter a soft lock state 522 in response to the counter reaching the predetermined cycle limit 520. The soft lock state 522 can include shutting down the operation of the furnace 100 for a period of time. In an example, period of time can comprise about one hour.

In an example, the controller 116 can initiate a different sequence if the supplemental sensor 114 ceases to transmit a signal indicating the fan of the inducer 104 is rotating at about a rotational threshold. If the supplemental sensor 114 fails to detect fan rotation 506 for a predetermined duration of time, such as about 12 seconds, while attempting to start the inducer 104, then the inducer can be de-energized 528 for an inducer shut down period 530. In an example, the shut down period 530 can comprise about five-minutes. Alternatively, if the inducer is running and the supplemental sensor 114 fails to detect fan rotation 506 for a predetermined duration of time, the inducer 104 can be de-energized 528. In an example, the predetermined duration of time can comprise about one second. The controller 116 will not attempt to re-start the inducer 104 until the inducer shut down period 530 has passed.

In an example, if a pressure signal indicating proper inducer operation is received 524 and the supplemental sensor 114 detects proper fan rotation 506, at any time while the thermostat 118 is calling for heat 502, then the controller 116 can initiate a sequence to initiate or continue to run 526 the furnace 100. In an example, a controller 116 for the furnace 100 can include an electrical circuit, such as on a printed circuit board assembly. The printed circuit can include a processor operatively coupled to a machine readable medium with instructions stored thereon. The controller 116 can include a power supply line, a transformer for reducing the supply voltage to an appropriate voltage, and electrical circuits connecting the inducer motor 304, igniter, thermostat 118, limit switches 608, pressure sensor 112, supplemental sensor 114, gas valves, and flame sensor. In an example, the controller 116 can be configured to operate the furnace 100 as depicted in FIGS. 4 and 5.

The controller 116 can be configured to start the furnace 100, such as in response to a call for heat from the thermostat 118. The supplemental sensor 114 can be positioned along the circuit connecting the controller 116 to the gas valves. The supplemental sensor 114 can include a switch, such as a switch that can close the circuit in response to detecting the rotation of the fan exceeding a first threshold speed. Ignition of the one or more burners 102 can only be initiated by the controller 116 if the switch of the supplemental sensor 114 is closed allowing the controller 116 to transmit a signal to open the gas valve.

Additionally or alternatively, the controller 116 can stop the heating of the enclosed space 120 with the furnace 100, such as in response to the elimination of a call for heat from the thermostat 118, the pressure sensor 112 or supplemental sensor 114 detecting an un-safe condition such as insufficient exhaustion of the products of combustion. The call for heat can be eliminated by opening a switch in the thermostat 118. The switch can be opened by an automatic thermostat control or by a user manually manipulating the switch to disable the call for heat. In an example, a gas valve cannot be opened if there is no closed switch in the thermostat 118. The gas valves can be operated by solenoids which are normally closed, such as when no power is supplied to the valve. The gas valves can be opened when power is supplied. Power to the gas valves can also be cut off by the supplemental sensor 114, such as if the fan speed is below the second threshold value causing a switch to open in the relay of the supplemental sensor 114. The pressure switch 112 can be closed in response to detecting a threshold pressure within the exhaust path 106. If the pressure switch 112 remains open when a prescribed burn time 512 expires, the controller can close the one or more gas valves. The furnace 100 can include a rollout or high-limit switch 608. The limit switch 608 can be positioned in series with the one or more gas valves and the supplemental sensor 114 in a circuit of the controller 116. The limit switch 608 can de-energize the at least one valve by opening the circuit in response to the temperature of the furnace 100 exceeding a threshold value for a given location within the furnace 100 in the heat exchanger 110 or in the proximity of the gas valves.

Additionally, the controller can be configured to start or stop the inducer motor 304, such as to exhaust the products of combustion generated when the furnace 100 is running.

Each of these non-limiting examples can stand on its own, or can be combined in any permutation or combination with any one or more of the other examples.

The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the present subject matter can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein.

In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.

Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.

The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the present subject matter should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 

What is claimed is:
 1. A gas furnace, comprising: a burner operable to produce an exhaust containing products of combustion; an exhaust path for receiving exhaust from the burner; a primary heat exchanger having an inlet operably connected to the exhaust path and an outlet operably connected to the exhaust path, wherein the primary heat exchanger is operably receive exhaust from the exhaust path and return exhaust to the exhaust path; a working fluid stream operably connected to the primary heat exchanger to receive heat from the exhaust received within the primary heat exchanger; an inducer, including a fan, operable to exhaust the products of combustion through the exhaust path; a pressure sensor configured to measure pressure within the exhaust path and generate a pressure sensor signal; a supplemental sensor configured to detect rotation of the fan and generate a detection signal indicating rotation of the fan; a controller communicatively connected to the burner, the inducer, the pressure sensor, and the supplemental sensor, the controller being configured to: rotate the fan of the inducer, receive the detection signal from supplemental sensor, operate burner upon receiving the detection signal from the supplemental sensor, and extinguish the burner if the pressure sensor signal is not received within a predetermined burn time.
 2. The gas furnace of claim 1, wherein the controller is configured to: increment a counter after each extinguishing of the burner.
 3. The gas furnace of claim 2, wherein the controller is configured to: ignite the burner in response to detecting rotation of the inducer fan from the supplemental sensor if the incremental counter value is less than a threshold; extinguish the burner if the pressure sensor signal is not received within a predetermined burn time.
 4. The gas furnace of claim 3, wherein the threshold is five.
 5. The gas furnace of claim 3, wherein the controller is configured to: maintain ignition of the burner and rotation of the fan of the inducer upon receiving a pressure sensor signal from the pressure sensor.
 6. The gas furnace of claim 5, wherein the controller is configured to: extinguish the burner if the detection signal from the supplemental sensor ceases.
 7. The gas furnace of claim 6, wherein the controller is configured to: prevent ignition of the burner when the counter exceeds a threshold cycle limit.
 8. The gas furnace of claim 7, wherein the controller is configured to: start a timer upon initiating rotation of the fan of the inducer.
 9. The gas furnace of claim 8, wherein the controller is configured to: ignite the burner when the timer exceeds a predetermined purge time.
 10. The gas furnace of claim 9, wherein the purge time is 30 seconds.
 11. The gas furnace of claim 9, wherein the controller is configured to: continue rotation of the fan of the inducer for a post-purge period of time after the burner is extinguished.
 12. The gas furnace of claim 11, wherein the controller is configured to: stop rotation of the fan of the inducer following an inducer shut down time.
 13. The gas furnace of claim 1, wherein the supplemental sensor transmits the detection signal when rotation of the fan exceeds a first threshold speed.
 14. The gas furnace of claim 13, wherein the supplemental sensor comprises a switch configured to close a circuit when rotation of the fan exceeds the first threshold speed.
 15. The gas furnace of claim 14, wherein the supplemental sensor comprises one of a centrifugal switch, a sail switch, a Hall effect switch activating when a rotational speed of the inducer motor exceeds a threshold, or a current sensing relay activating when a motor current of the inducer exceeds a threshold.
 16. The gas furnace of claim 15, wherein the supplemental sensor includes a hysteresis such that transmission of the detection signal continues until rotation of the inducer fan is less than a second threshold, the second threshold being lower than the first threshold.
 17. The gas furnace of claim 16, wherein the first threshold is 2,000 revolutions per minute.
 18. The gas furnace of claim 1, wherein the pressure sensor transmits the pressure sensor signal when the measured pressure within the exhaust path exceeds a threshold pressure.
 19. The gas furnace of claim 18, wherein the pressure sensor includes a switch configured to close a circuit and thereby transmit the pressure sensor signal to the controller when the measured pressure within the exhaust path exceeds the threshold pressure.
 20. The gas furnace of claim 19, wherein the pressure sensor comprises one of a diaphragm switch, a differential switch, or a sail switch.
 21. The gas furnace of claim 19, wherein the pressure sensor is located in a port of a housing surrounding the fan of the inducer.
 22. The gas furnace of claim 1, wherein the burn time is 60 seconds.
 23. The gas furnace of claim 1, wherein the gas furnace comprises a condensing gas furnace.
 24. The gas furnace of claim 1, further comprising a secondary heat exchanger having an inlet proximate to an outlet of the primary heat exchanger and an outlet proximate an inlet of the inducer.
 25. A method for operation a condensing gas furnace with a thaw cycle, comprising: rotating a fan of an inducer; receiving a signal from a sensor indicating rotation of the fan of the inducer; initiating an ignition sequence for a burner upon receiving the Hall Effect signal; operating the burner for a first period of time; monitoring for a pressure sensor signal; ceasing operation of the burner if the pressure sensor signal is not received within a second period of time; repeating operation of the burner for the first period of time for a specified number of times until pressure sensor signal is detected.
 26. A condensing gas furnace with a thaw cycle, comprising: a primary heat exchanger; a gas manifold assembly including at least one burner and at least one gas valve, wherein the gas manifold assembly is positioned at the inlet of the primary heat exchanger; an igniter for initiating combustion of fuel dispensed from the gas manifold assembly; a secondary heat exchanger coupled to the outlet of the primary heat exchanger; an inducer having a fan coupled the secondary heat exchanger, wherein the fan is rotatable to generate airflow through the first and secondary heat exchangers; a pressure switch located in-line with the inducer; a sensor for detecting the angular velocity of the fan the inducer; and a controller configured to: start an inducer, receive a signal velocity from the sensor, the sensor indicating that the rotational velocity of the fan exceeds a predetermined rotational, initiate an ignition sequence for the burner upon receiving the signal, operate the burner for a first period of time, monitor for a pressure sensor signal; cease operation of the burner if the pressure sensor signal is not received within a second period of time; repeat operation of the burner for the first period of time for a specified number of times until pressure sensor signal is detected. 